And natural and organic distributor to Whole Foods, United Natural Foods (UNFI), posted a strong third quarter net sales result, up 11.8 percent to US$2.65 billion (13 weeks to April 28), boosted by a 24.3 percent year-on-year growth to its Whole Foods "supernatural channel."

Amazon has rapidly implemented changes in its retail operations since buying Whole Foods last year. Some high-level Whole Foods executives have left, and Amazon has altered relationships with suppliers, CNBC.com said.

CEO and co-founder John Mackey is working alongside two Amazon executives, Rosanna Godden and Heather Dystrup-Chiang, to ensure a smooth transition. Ms. Godden, at Amazon since 2012, became vice president of strategic business integration at Whole Foods in March. Ms. Dystrup-Chiang, at Amazon for seven years, is now director of product management for Whole Foods.

And Amazon is up for an additional $22 billion in contractually obligated future purchases. The purchase commitments, which Amazon describes as "firm, non-cancellable," appeared in Whole Foods' financial statement in November and are almost entirely tied to its contract with its largest supplier, UNFI, CNBC said.

The commitment runs until 2025, an unusually long tie-up for Amazon, known for signing short-term contracts with its suppliers.

United Natural Foods' chairman and CEO Steven L. Spinner said that UNFI delivered strong results for the quarter underscoring the strength of its market position as it continues to grow amid a continuous change in the retail landscape.

"UNFI continues to be an important connector between manufacturers, brick and mortar retailers as well as eCommerce customers. We are pleased with the continued momentum in our business, and we continue to work to balance and improve upon the challenges associated with this higher-than-expected growth," Mr. Spinner said.

Gross margin for fiscal 3Q 2018 was 15.41 percent, reflecting a five-basis point decrease over last fiscal year's third quarter. "Gross margin for the third quarter included the positive impact of a change in accounting estimate of $20.9 million as the company revised its calculation for its accrual for inventory purchases," UNFI said.

The success of UNFI's build-out-the-store strategy is driving growth in new categories, most notably, health, beauty, and supplements. The approach also focuses on distributing more fresh and specialty foods and helped drive sales to independent natural food stores and conventional supermarkets.